1. Field of the Invention
The present invention relates to an optical information processing apparatus in which a focused light beam is irradiated onto an optical recording medium and information is recorded and/or reproduced.
2. Related Background Art
Hitherto, apparatuses for recording or reproducing a signal by using a laser beam have been put into practical use. For example, a compact disc (CD) player, a laser disc (LD) player, and the like have been known. In these apparatuses, a signal is recorded as pits 2 on a disk 1 as shown in FIG. 1. To read the information signal from the disk 1, the light beam is projected onto the disk 1 and optically scans to thereby read the pits 2. That is, when tracks on which the pits 2 are formed are scanned by the light beam, an intensity of the reflected light changes in correspondence to the presence or absence of the pit, so that the information pits can be optically read.
On the other hand, there has also been developed a magneto-optical disk apparatus in which a light beam is projected onto a magneto-optical disk on which tracks are spirally or concentrically formed and the information signal is optically recorded, reproduced, or erased. FIG. 2 shows a construction of a magneto-optical disk 3 which is used in such an apparatus.
In the diagram, reference numeral 4 denotes, for instance, a track guide groove formed spirally on the disk. The magneto-optical signal is recorded in the central region between the track guide groove and the adjacent track guide groove. Reference numeral 5 denotes preformat bits indicative of a detection pattern such as track addresses, sector addresses, sector marks, and the like in the case of using the magneto-optical disk as a disk for data information or the like. The preformat pits 5 are formed in the central region between the track guide groove and the adjacent track guide groove. An address is read by the preformat pit 5 and the data corresponding to the address is reproduced, recorded, or erased by a magneto-optical signal in the circumferential direction for a period of time until the next preformat pit 5.
In the case where the tracks are spirally formed on the disk, the tracks are constructed as a single long track. However, in the invention, it is assumed that the track of one circumference of a disk if formed and a number of such tracks are arranged in parallel in the radial direction.
In a tracking servo system for allowing a light beam to trace the tracks by using the track guide grooves 4 on the disk 3 as mentioned above, a reflectance of the disk surface deteriorates at the location of the preformat pit and, further, an S/N ratio of a tracking error signal deteriorates, and a kind of noise is generated. If an amount of the preformat pits occupies about 4 to 5% of the area on all of the track guide grooves and when the tracking servo system is in the operative mode, a degree of tracing fidelity of the tracks slightly deteriorates and a situation such that the tracking servo becomes ineffective does not occur.
However, for instance, in the case of jumping to the adjacent track and reproducing data from the adjacent track as disclosed in U.S. Pat. No. 4,057,832, there is executed a sequence such that the tracking servo is made inoperative, the light beam jumps to the next adjacent track, and when the light beam arrives at a predetermined position on the next adjacent track, the tracking servo system is again made operative. In such a case, it is necessary to raise the gain immediately after the tracking servo is made operative, thereby enabling the tracking servo to be easily pulled in. However, in the case of making the tracking servo operative at the position of the preformat pit, the noise of the preformat pit is also amplified, so that there is a problem such that the pull-in of the tracking servo becomes unstable.